Using subdomains in diffusion equation

[dokken]

You need to use boolean fragments to resolve the interface between the domains, see for instance:

Gmsh 4.11.1

or

Transitioning from mesh.xml to mesh.xdmf, from dolfin-convert to meshio

@Iactor, the problem with your code is your mesh. It is not a connected mesh, and consists of several bits and pieces. In particular, you add extra surfaces over the surfaces defined in the surface mesh. I’ve rewritten your code, using boolean fragments in gmsh to generate what I think is the desired mesh. Note that in this mesh, elements align over boundaries, which was not the case in your original code. // Gmsh project created by Jørgen S. Dokken 2020 SetFactory("OpenCASCADE"); Box(1) = {0,…

or
https://jsdokken.com/converted_files/tutorial_pygmsh.html